This invention relates to a circuit for controlling the position of the throttle of an engine and, more particularly, to an electronic drive-by-wire throttle position control circuit for an engine that provides two output resolutions from a single input command signal.
In an electronic drive-by-wire system for an automotive vehicle engine, the motorized throttle must produce the same dynamic range of air flow as the conventional cable drive, while retaining the resolution required for idle speed control previously produced by the idle air control valve. In general, to accurately control throttle at idle, much more resolution is needed than at other throttle positions. If the slope of the throttle angle versus the command voltage is established to obtain the required throttle angle range for the available command voltage range, the resolution for idle speed control may be inadequate. Conversely, if the slope is adjusted to obtain the required resolution for idle speed control, then the range of throttle angle control may not be achievable for the available control voltage.
A problem exists in conventional systems in that the microprocessor signal used to generate the drive signal has a limited bit resolution. For example, where 8-bit resolution (256 bits of information) is sufficient to command the throttle range, much finer resolution such as 10-bit resolution (1024 bits of information or four times the resolution of 8 bits) is required to command the idle speed range. However, the conventional vehicle engine control module has neither a 10-bit digital to analog converter nor a 128 Hertz 10-bit pulse width modulator signal. The conventional vehicle engine control only has an 8 bit 128 Hertz pulse width modulator signal. Consequently, the engine control modules of existing systems do not have the resolution capabilities required for idle speed range.